1. Field of the Invention
This invention relates to apparatus and methods for encoding or embedding and decoding or extracting information in analog signals, such as audio, video and data signals, either transmitted by radio wave transmission or wired transmission, or stored in a recording medium such as optical or magnetic disks, magnetic tape, or solid state memory.
2. Background and Description of Related Art
The present invention is concerned with techniques for embedding and extracting auxiliary information within an existing signal, such as an audio or video signal.
An area of particular interest to certain embodiments of the present invention relates to the market for musical recordings. Currently, a large number of people listen to musical recordings on radio or television. They often hear a recording which they like enough to purchase, but don""t know the name of the song, the artist performing it, or the record, tape, or CD album of which it is part. As a result, the number of recordings which people purchase is less than it otherwise would be if there was a simple way for people to identify which of the recordings that they hear on the radio or TV they wish to purchase.
Another area of interest to certain embodiments of the invention is copy control (also referred to as digital watermarking). There is currently a large market for audio software products, such as musical recordings. One of the problems in this market is the ease of copying such products without paying those who produce them. This problem is becoming particularly troublesome with the advent of recording techniques, such as digital audio tape (DAT), which make it possible for copies to be of very high quality. Thus it would be desirable to develop a scheme which would prevent the unauthorized copying of audio recordings, including the unauthorized copying of audio works broadcast over the airwaves. It is also desirable for copyright enforcement to be able to insert into program material such as audio or video signals digital copyright information identifying the copyright holder, which information may be detected by appropriate apparatus to identify the copyright owner of the program, while remaining imperceptible to the listener or viewer.
Yet another field of interest relating to the present invention pertains to automatic royalty tracking and proof of performance of copyrighted material or commercial advertisements, by which copyright owners are able to track public performances or broadcasts of their material for royalty payment purposes, and advertisers are able to confirm that. commercials which they have paid for were actually broadcast at the proper time and date.
Still another area of interest to the present invention relates to integrity verification or tampering detection, wherein the creator of an audio or audiovisual work can determine whether it has been altered, modified or incorporated into another work.
Various prior art methods of encoding additional information onto a source signal are known. For example, it is known to pulse-width modulate a signal to provide a common or encoded signal carrying at least two information portions or other useful portions. In U.S. Pat. No. 4,497,060 to Yang (1985) binary data is transmitted as a signal having two differing pulse-widths to represent logical xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d (e.g., the pulse-width durations for a xe2x80x9c1xe2x80x9d are twice the duration for a xe2x80x9c0xe2x80x9d). This correspondence also enables the determination of a clocking signal.
With respect to systems in which audio signals produce audio transmissions, U.S. Pat. Nos. 4,876,617 to Best et al. (1989) and 5,113,437 to Best et al. (1992) disclose encoders for forming relatively thin and shallow (e.g., 150 Hz wide and 50 dB deep) notches in mid-range frequencies of an audio signal. The earlier of these patents discloses paired notch filters centered about the 2883 Hz and 3417 Hz frequencies; the later patent discloses notch filters but with randomly varying frequency pairs to discourage erasure or inhibit filtering of the information added to the notches. The encoders then add digital information in the form of signals in the lower frequency indicating a xe2x80x9c0xe2x80x9d and in the higher frequency a xe2x80x9c1xe2x80x9d. In the later Best et al. patent an encoder samples the audio signal, delays the signal while calculating the signal level, and determines during the delay whether or not to add the data signal and, if so, at what signal level. The later Best et al. patent also notes that the xe2x80x9cpseudo-random mannerxe2x80x9d in moving the notches makes the data signals more difficult to detect audibly.
Other prior art techniques employ the psychoacoustic model of the human perception characteristic to insert modulated or unmodulated tones into a host signal such that they will be masked by existing signal components and thus not perceived. See. e.g. Preuss et al., U.S. Pat. No. 5,319,735, and Jensen et al., U.S. Pat. No. 5,450,490. Such techniques are very expensive and complicated to implement, while suffering from a lack of robustness in the face of signal distortions imposed by perception-based compression schemes designed to eliminate masked signal components.
The prior art fails to provide a method and an apparatus for embedding and extracting auxiliary analog or digital information signals onto analog audio or video frequency signals for producing humanly perceived transmissions (i.e., sounds or images) such that the audio or video frequency signals produce substantially identical humanly perceived transmission prior to as well as after encoding with the auxiliary signals (in other words, the embedded information is transparent to the listener or viewer), which is also robust to a high degree of signal distortions caused by noisy transmission mediums, etc. The prior art also fails to provide relatively simple and inexpensive apparatus and methods for embedding and extracting signals defining auxiliary information into audio or video frequency signals for producing humanly perceived audio transmissions.
The present invention provides apparatus and methods for embedding or encoding, and extracting or decoding, auxiliary (analog or digital) information in an analog host or cover signal in a way which has minimal impact on the perception of the source information when the analog signal is applied to an appropriate output device, such as a speaker, a display monitor, or other electrical/electronic device.
The present invention further provides apparatus and methods for embedding and extracting machine readable signals in an analog cover signal which control the ability of a device to copy the cover signal.
In summary, the present invention provides for the encoding or embedding of an auxiliary signal in an analog host or cover signal, by generating a replica signal from the cover signal, modifying the replica signal as a function of the auxiliary signal, and inserting the modified replica signal back into the analog cover signal to provide a stego signal. The invention further provides for the extraction of embedded auxiliary signals from stego signals by generating a replica of the stego signal, and correlating the replica with the stego signal.
According to another aspect of the invention, apparatus for embedding and extracting auxiliary signals in an analog cover signal, is provided, comprising a replica generator for generating a replica signal from the cover signal, a modulator for modifying the replica signal as a function of the auxiliary signal, an adder for inserting the modified replica signal back into the analog cover signal to produce a stego signal, a receiver for receiving the stego signal, a generator for generating a replica signal from the stego signal, a modulator for modifying the received stego signal as a function of the replica signal of the received stego signal, and an extractor for extracting the auxiliary signal by filtering the modified received stego signal.
The term cover signal as used hereinafter refers to a host or source signal, such as an audio, video or other information signal, which carries or is intended to carry embedded or hidden auxiliary data.